The so-called “open-type diaper” which includes a diaper body and a girth portion provided on a side edge of one of the front side and the back side of the diaper body has been known in the art as a disposable worn article. The open-type diaper has an advantage that it can be used in the form of either a pants-type or a tape-type by fastening in advance the girth portion to the other one of the front side and the back side with a fastener. A method for fastening the girth portion of such a diaper to the diaper body with a fastener in advance so that it is provided as a pants-type has been proposed in the art (see the first patent document).